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The association between stratospheric weak polar vortex events and cold air outbreaks in the Northern Hemisphere

  • NORCE, Bergen, Norway
  • Met Office and University of Exeter


Previous studies have identified an association between temperature anomalies in the Northern Hemisphere and the strength of stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool during the mature and late stages of weak vortex events in the stratosphere. A substantial part of the temperature changes are associated with changes in the Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure patterns in the troposphere. The apparent coupling between the stratosphere and the troposphere may be of relevance for weather forecasting, but only if the temporal and spatial nature of the coupling is known. Using 51 winters of re-analysis data, we show that the development of the lower-tropospheric temperature relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including the formation of geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly over northwest Eurasia is associated with long-lived and robust cold anomalies over Asia and Europe. A few weeks later, near the mature stage of the weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America. The probability of cold air outbreaks increases by more than 50% in one or more of these regions during all phases of the weak vortex events. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. As large changes in the frequency of extremes are involved, this process is important for the medium-range and seasonal prediction of extreme cold winter days. Three-hundred-year pre-industrial control simulations by 13 coupled climate models corroborate our results. Copyright © 2010 Royal Meteorological Society and Crown Copyright.
Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 886893, April 2010 Part B
The association between stratospheric weak polar vortex events
and cold air outbreaks in the Northern Hemisphere
Erik W. Kolstad,a*TarjeiBreiteig
a,c and Adam A. Scaifeb
aBjerknes Centre for Climate Research, Bergen, Norway
bHadley Centre for Climate Prediction and Research, Met Office, Exeter, UK
cGeophysical Institute, University of Bergen, Bergen, Norway
*Correspondence to: Erik W. Kolstad, Bjerknes Centre for Climatic Research, Postboks 7810, 5020 Bergen, Norway.
The Contribution of Adam A. Scaife was written in the course of his employment at the Met Office, UK and is published
with the permission of the Controller of HMSO and the Queen’s Printer for Scotland.
Previous studies have identified an association between temperature anomalies
in the Northern Hemisphere and the strength of stratospheric polar westerlies.
Large regions in northern Asia, Europe and North America have been found to
cool during the mature and late stages of weak vortex events in the stratosphere.
A substantial part of the temperature changes are associated with changes in the
Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure
patterns in the troposphere. The apparent coupling between the stratosphere and the
troposphere may be of relevance for weather forecasting, but only if the temporal and
spatial nature of the coupling is known. Using 51 winters of re-analysis data, we show
that the development of the lower-tropospheric temperature relative to stratospheric
weak polar vortex events goes through a series of well-defined stages, including the
formation of geographically distinct cold air outbreaks. At the inception of weak
vortex events, a precursor signal in the form of a strong high-pressure anomaly over
northwest Eurasia is associated with long-lived and robust cold anomalies over Asia
and Europe. A few weeks later, near the mature stage of the weak vortex events,
a shorter-lived cold anomaly emerges off the east coast of North America. The
probability of cold air outbreaks increases by more than 50% in one or more of these
regions during all phases of the weak vortex events. This shows that the stratospheric
polar vortex contains information that can be used to enhance forecasts of cold air
outbreaks. As large changes in the frequency of extremes are involved, this process
is important for the medium-range and seasonal prediction of extreme cold winter
days. Three-hundred-year pre-industrial control simulations by 13 coupled climate
models corroborate our results. Copyright c
2010 Royal Meteorological Society
and Crown Copyright.
Key Words: stratosphere-troposphere interactions; climate models; natural variability; North Atlantic
Oscillation; Arctic Oscillation
Received 29 May 2009; Revised 11 February 2010; Accepted 2 March 2010; Published online in Wiley InterScience
4 May 2010
Citation: Kolstad EW, Breiteig T, Scaife AA. 2010. The association between stratospheric weak polar
vortex events and cold air outbreaks in the Northern Hemisphere. Q. J. R. Meteorol. Soc. 136: 886– 893.
Copyright c
2010 Royal Meteorological Society and Crown Copyright.
Weak Polar Vortex and Cold Air Outbreaks 887
1. Introduction
Cold air outbreaks (CAOs) are departures of cold air masses
into warmer regions. Over land, these events can lead to
deaths and damage (Mercer, 2003; Barnett et al., 2005;
Pinto et al., 2007). Over the ocean, CAOs are important for
a number of reasons: they give rise to mesoscale weather
phenomena such as polar lows (Bracegirdle and Gray, 2008),
they lead to enhanced heat and momentum fluxes from the
ocean to the air (Renfrew and Moore, 1999) and may
therefore influence the ocean circulation (Pickart et al.,
2003), and they cause rapid formation of sea ice in marginal
ice zones (Skogseth et al., 2004). In recent years it has
emerged that anomalies in the stratospheric circulation can
be associated with tropospheric CAOs (Thompson et al.,
2002; Cai and Ren, 2007; Scaife et al., 2008).
Normally, the extratropical stratosphere is characterised
by a strong westerly circumpolar flow. In winter, planetary
waves of tropospheric origin propagate continuously into
the stratosphere (Charney and Drazin, 1961), where they
break and exert a drag on the zonal flow (McIntyre and
Palmer, 1983; Polvani and Waugh, 2004). This violates
the geostrophic balance and induces a poleward drift of
air masses. At high latitudes, the air converges, sinks and
warms adiabatically. If there is severe wave-breaking, the
stratospheric zonal flow reverses, giving rise to stratospheric
sudden warmings (SSWs: Matsuno, 1971), which may last
for days to weeks (Limpasuvan and Hartmann, 1999).
After their first appearance in the upper stratosphere,
circulation anomalies are occasionally found at successively
lower levels (Matsuno, 1970; Lorenz and Hartmann, 2003).
After reaching the tropopause, the anomalies may impact
the troposphere through an interaction with synoptic-scale
eddies (Song and Robinson, 2004), or more directly through
induced meridional circulations. As a result, a negative
Northern Annular Mode (NAM: Thompson and Wallace,
2001) and North Atlantic Oscillation (NAO: Hurrell et al.,
2003) pattern may occur near the surface some weeks after
the first warming signal in the upper stratosphere (Baldwin
and Dunkerton, 2001; Baldwin et al., 2003; Limpasuvan
et al., 2004).
Negative NAM and NAO regimes in the troposphere
have a profound influence on the weather in large and
widespread regions of the Northern Hemisphere (NH)
(Kenyon and Hegerl, 2008). Atlantic and Pacific storm tracks
shift latitudinally (Hurrell and Van Loon, 1997; Baldwin and
Dunkerton, 2001), Greenland and Newfoundland warm
(Thompson et al., 2002), and the frequency and severity of
CAOs increase over large parts of east Asia (Chen et al.,
2005; Jeong and Ho, 2005), northern Eurasia (Scaife et al.,
2008) and eastern North America (Thompson and Wallace,
2001; Walsh et al., 2001; Cellitti et al., 2006). Over the ocean,
negative phases of the NAO, and positive height anomalies
over Greenland in particular, are associated with marine
CAOs over the Nordic Seas (Kolstad et al., 2009).
Motivated by the link that has been observed between
anomalous stratospheric events and the tropospheric
climate, we aim to provide a detailed description of
tropospheric cold anomalies in relation to such events.
Thompson et al. (2002) investigated the mean temperature
response during the first 60 days after the onset dates of
stratospheric anomalous vortex conditions. Here, we extend
their work by assessing the temperature development and
changes in the probability of CAOs at different stages
Table I. The official CMIP3 designations of the models that
were used in this study.
of stratospheric weak vortex events. We find that the
tropospheric temperature development goes through several
distinct and well-defined stages of stratospheric weak vortex
events and we identify CAOs over both continental and
oceanic regions. These results are corroborated by data from
300-year time slices of 13 coupled model simulations.
2. Data and methods
Daily mean fields from the National Centers for Envi-
ronmental Prediction/National Center for Atmospheric
Research (NCEP/NCAR) re-analysis (hereafter referred to
as NNR) data (Kalnay et al., 1996) were used throughout
the study. The analysis period was from the autumn/winter
of 1958 to the winter/spring of 2009.
Monthly mean data from 13 models in the World
Climate Research Programme’s (WCRP’s) Coupled Model
Intercomparison Project phase 3 (CMIP3) multi-model
dataset were also used. An analysis of the stratospheric
variability of these models is found in Cordero and Forster
(2006). The models used are listed in Table I (note that
we used a more recent BCCR-BCM model simulation, as
described by Otter˚
aet al. (2009)). These were the only models
that included 300-year time slices of pre-industrial control
simulations, with no anthropogenic or natural forcing. The
time slices were chosen arbitrarily from the years that were
available for download.
The most commonly used measure of stratospheric
variability is the NAM index. However, as both the spatial
greatly across the models, we defined a Vortex Strength Index
(VSI) as ZP,where,ZP(Zcos ϕ)cos ϕ,Z=
ZZ,Zis the geopotential height, Zis its climatological
mean, ϕis the latitude, and the sum was performed on all
grid points north of 65N. The reason for the minus sign is
that the vortex is weak when the pressure is high and vice
versa. Anomalies were formed by removing the date-wise
climatological mean for each grid point. To ensure that the
climatology was smooth, we applied a 31-day running-mean
filter. The highest pressure level (below 10 hPa, where some
of the models appeared to be unreliable) for which data was
available from all the models was 50 hPa, so this was used
as the stratospheric reference level for both the models and
the NNR. Baldwin and Thompson (2009) have shown that
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
888 E. W. Kolstad et al.
Table II. SSW central dates since the ones identified by
Charlton and Polvani (2007), as derived from the NNR.
18 Jan 2003
7 Jan 2004
21 Jan 2006
24 Feb 2007
22 Feb 2008
24 Jan 2009
a VSI in this form is practically identical to the daily zonal-
mean NAM index. The monthly VSI was computed from
the model data in a similar way, although the climatological
monthly means were not smoothed.
The analysis of this paper focuses on the temporal
development of the signals of anomalous temperature and
geopotential height. Both parameters were evaluated at a
pressure level of 850 hPa and their date-wise climatological
means and anomalies were found in the same manner as
for the area-averaged 50 hPa geopotential height anomalies
described above, although for each grid point separately.
Our analysis is centred on composites of days and
months for which the stratospheric vortex is weak. We
define Weak vortex days (WVDs) in the NNR as the days
for which the daily VSI falls below its overall wintertime
(DecemberMarch) 10th percentile. An alternative to this
method would be to remove the seasonal cycle (by using
the date-wise climatological 10th percentile as a threshold
instead), but this would have forced the WVDs to be equally
distributed among the winter months. Cold days are defined
as days with an 850 hPa temperature below its date-wise
climatological 10th percentile. When identifying cold days
we did remove the seasonal cycle, as the purpose of defining
cold days is to assess whether a given day is colder than
‘normal’. Weak vortex months (WVMs) and Cold months
in the models are defined with respect to the overall 10th
percentiles of the monthly mean anomalies.
To assess the sensitivity of the results to the choice of
method, the analysis was also done by compositing with
respect to a set of SSW central dates, as defined and identified
by Charlton and Polvani (2007) (hereafter referred to as
CP07). Six SSWs have occurred since CP07, yielding a total
of 31 SSWs since 1958. The new central dates, as derived
from the NNR, are listed in Table II.
3. Results
3.1. Weak vortex events
In Figure 1(a), a matrix of VSI values for each day in the
analysis period is shown. The values were grouped with
respect to deciles. The blue days are the WVDs as defined
above. The SSW central dates are shown using crosses. As
mentioned earlier, due to the way they were computed, the
density of WVDs is higher in midwinter than in early and
late winter. Figure 1(a) shows that this complies well with the
seasonal distribution of the SSW central dates. An advantage
of CP07’s approach is that all their events are independent,
and the study of lead/lag processes is therefore free of the
effects of artificial smoothing. The composite zonal-mean
of the zonal wind at 10 hPa and 60N relative to the SSW
central dates is shown in Figure 1(b). The rapid weakening
and gradual recovery of the polar vortex is clear. However,
a disadvantage of CP07’s approach, or indeed any approach
that selects a specific reference date for each event, is that one
must be certain that the correct date has been chosen in each
case. Otherwise, the temporal signal may be distorted. The
relative scarcity of observed SSWs adds weight to this issue.
The VSI-based approach, in which each WVD is regarded as
a separate ‘event’, leads to runs of days and a smoothing of
the temporal signal. However, it is simple and sensitive to
only one apriorichoice: the selection of a threshold value for
WVDs. The composite 50 hPa polar cap geopotential height
anomalies relative to WVDs are shown in Figure 1(c). The
symmetrical evolution of the height anomalies about the
WVDs is a result of the smoothing introduced by the VSI-
based approach. The symmetry also shows that the WVD
approach is biased towards the middle date of longer events.
We also note that in composites of WVDs, persistent events
are given more weight than transient ones.
3.2. Tropospheric signature
In this section, we analyse temporal developments in the
troposphere throughout the life cycles of stratospheric weak
vortex events using both CP07’s approach and the VSI-
based approach, with an emphasis on cold anomalies. To
simplify the notation, we build loosely on the terminology
of Limpasuvan et al. (2004). They examined the evolution of
wave activity fluxes and atmospheric pressure fields in several
sub-periods of SSW life cycles. We define the following
phases of weak vortex events: Precursor (4531 days before
the central dates and WVDs), Onset (3016 days before
same), Growth (151 days before same), Peak (0 14 days
after same), Mature (1529 days after same), Decline
(3044 days after same) and Decay (45 59 days after
same). Note that the developments that are seen in Fig. 9
of Limpasuvan et al. (2004) are not necessarily directly
comparable to the developments in our time intervals.
In Figure 2, the development of the 850 hPa geopotential
height and temperature anomalies relative to both the SSW
central dates (Figure 2(a)) and WVDs (Figure 2(b)) are
shown. In the early stages (Precursor, Onset and Growth),
positive height anomalies centred over northwest Eurasia
and negative anomalies near the Bering Strait are found.
This corresponds to a pattern that has been found to
favour stratospheric warmings through an enhancement of
upward-propagating tropospheric wave-number-one waves
(Kuroda and Kodera, 1999; Garfinkel et al., 2010). It
is therefore thought to be a tropospheric precursor of
warmings aloft. Cold anomalies arise over north-eastern Asia
through anomalous northerly wind components, resulting
in southward advection of cold, Arctic air masses. Cold
anomalies are also found over Europe, where there are
anomalous easterlies. These winds relate to the anomalous
ridge over northwest Eurasia. To our knowledge, these
cold anomalies, which appear too early to be affected by
downward propagation of the negative NAM-like signals
after SSWs, have not been documented previously in
relation to weak vortex events. Bueh and Nakamura (2007)
document similar temperature patterns in response to
the so-called Scandinavian Pattern, which resembles the
anomalous height pattern found in Figure 2(b) (Precursor
By the Growth and Peak stages in the WVD framework
(Figure 2(b)), an NAO-like anomaly has appeared. This
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
Weak Polar Vortex and Cold Air Outbreaks 889
Days relative to SSW central dates
Zonal wind (ms-1)
P anomaly (m)
30 20 10 0 10 20 30
Days relative to WVDs
30 20 10 0102030
1 Dec 1 Jan 1 Feb 1 Mar
Figure 1. (a) The daily VSI, sorted and grouped with respect to deciles. The SSW central dates based on the algorithm of Charlton and Polvani (2007) are
marked with crosses. (b) The composite daily mean zonally averaged zonal wind at 60N and 10 hPa on each day relative to SSW central dates. (c) The
composite daily polar-cap 50 hPa geopotential height (ZP) anomalies on each day relative to WVDs.
2.5 1.5 0.5 0.5 1.5 2.5
PeakGrowthOnsetPrecursor Mature Decline Decay
Figure 2. Composites of 850 hPa geopotential height anomalies (in m with solid contours, positive in black, negative in grey, contour interval 10 m,
zero contour omitted) and 850 hPa temperature anomalies (in K with filled contours, with white contours along the values specified on the colour bar)
relative to (a) SSW central dates and (b) WVDs, averaged over the specified time intervals. The region shown is the Northern Hemisphere north of 30N,
with Eurasia to the right and North America to the left.
leads to an anomalous northerly flow and cold anomalies
in northern Europe. This is consistent with an increased
frequency of marine CAOs in the Nordic Seas region
under negative NAO conditions (Kolstad et al., 2009).
As the pressure anomalies are contained primarily in the
Atlantic sector by this time, the cold anomalies in Asia
diminish in magnitude. At the same time, cold anomalies
appear on the east coast of North America. Corresponding
warm anomalies over Canada and the Mediterranean/North
Africa complete the quadrupole pattern of temperature
anomalies that are associated with the NAO (Stephenson
and Pavan, 2003). In the Mature, Decline and Decay stages,
the NAO pattern gradually weakens, and the most prominent
cold anomalies are found in Asia and Europe. This is
consistent with findings from previous studies (Baldwin
and Dunkerton, 2001; Thompson et al., 2002; Chen et al.,
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2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
890 E. W. Kolstad et al.
0 0.25 0.5 0.75 1.25 1.5 1.75 2 2.25 2.5
PeakGrowthOnsetPrecursor Mature Decline Decay
Figure 3. The composite number of cold days divided by the date-wise climatological mean number of cold days for each grid point relative to (a) SSW
central dates and (b) WVDs, during the specified time intervals. Otherwise the plotting conventions are as in Figure 2.
0 0.25 0.5 0.75 1.25 1.5 1.75 2 2.25 2.52.5 1.5 0.5 0.5 1.5 2.5
Mature +
Growth +
Precursor +
Mature +
Growth +
Precursor +
Figure 4. (a), (b) Plotting conventions as in Figure 2. (c), (d) Plotting conventions as in Figure 3. The composite averages were computed relative to
WVDs for the two halves of the NNR period, with the period 1959–1983 in (a), (c) and 1984– 2009 in (b), (d).
To summarise, Figure 2 shows temporally and geograph-
ically distinct cold anomalies throughout the life cycle of
weak vortex events. The cold anomalies were found using
the CP07 method (Figure 2(a)) and using the WVD method
(Figure 2(b)), although the exact timing and relative ampli-
3.3. Relative frequency of stratosphere-related cold air
Figure 2 was based on changes to the mean temperature
field, with no regard to its extreme values. We now define
the quantity αas the fractional change in the number
of cold days (days for which the 850 hPa temperature
anomaly falls below its date-wise 10th percentile) with
respect to climatology. If α=1.5, cold days are 50%
more likely than normal. As this parameter is only
concerned with cold extremes, the evolution of α,which
is shown in Figure 3, provides a useful complement to
Figure 2.
In the early stages of weak vortex events, i.e. during the
Precursor and Onset stages, α>1.5 most consistently in
Asia and Europe. By the Growth stage, α>1.5offthecoast
of North America in the WVD framework. In the later
Mature and Decline stages, the strongest cold anomalies are
again confined to Asia and Europe. It is only in Asia that
α>1.75 during all the periods shown. The large fractional
changes in Figure 3 show that the frequency of cold days
is affected strongly around the time of sudden stratospheric
3.4. Robustness of the results
In Figure 4, the same analysis that was used to produce
Figures 2(b) and 3(b) was applied to the two halves of the
analysis period. Note that we averaged over longer time
periods than in Figures 2 and 3. In general, the features
shown previously for both the mean and extreme events are
robust to this halving of the data period. High pressure is
observed over northwest Eurasia prior to the weak vortex
events, and a subsequent negative NAO-like pattern is seen.
Similarly, cold anomalies are found over Asia, Europe and
near the east coast of North America. The perhaps largest
difference between the two periods is found over the Pacific
in the Mature/Decline phases. In the first part of the period,
an anomalous low over the Aleutian Islands (Figure 4(a)) is
associated with advection of cold, continental air out over
the Pacific (Figure 4(c)). The large Pacific high anomaly in
the latter part of the period (Figure 4(b)) is associated with
Copyright c
2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
Weak Polar Vortex and Cold Air Outbreaks 891
a 75% increase in the number of cold days along the west
coast of North America (Figure 4(d)).
As an additional test of robustness, we examined monthly
mean data from 13 coupled climate models. This part of
the analysis is done using the WVM framework. Some
of the CMIP3 models have low model tops and many of
them underestimate the stratospheric variance (Cordero
and Forster, 2006). The models will therefore be used to
evaluate the temporal and spatial variability rather than the
exact amplitudes of the anomalies. Based on the symmetry
of Figure 1(c), it is natural to define that WVMs correspond
to the Growth and Peak phases of weak vortex events.
Figure 5(a) shows the 850 hPa geopotential height and
temperature anomalies during WVMs (Growth/Peak phase),
as well as during the preceding (Precursor/Onset phase) and
the succeeding (Mature/Decline phase) months. The initial
cold anomaly in Asia, the westward shift of the northwest
Eurasia warm anomaly and the appearance of cold anomalies
in Europe and off the east coast of North America are all
seen in the model ensemble around weak vortex months.
The temperature developments are consistent with the
anomalous northerly and easterly flow that is associated
with the pressure anomalies, whereas the overall westward
progression of the temperature pattern indicates further
potential for seasonal predictability.
In Figure 5(b), the changes to the probability of cold
months in the different stages are shown. The following cold
anomalies are associated with a higher than 50% increase
in the number of cold months: (1) The cold anomaly over
Asia in the Precursor/Onset and Growth/Peak phases, (2) the
cold anomaly over northern Europe in the Growth/Peak
and Mature/Decline phases, and (3) the cold anomaly over
north-eastern North America in the Growth/Peak phases.
Qualitatively, the features in Figure 5 are in good agreement
with Figures 2 and 3, although the magnitudes of the
anomalies are generally weaker. This is at least partly due to
the much larger sample size of the model data.
4. Concluding remarks
The relationship between stratospheric weak vortex events
and tropospheric developments, and cold air outbreaks
(CAOs) in particular, were investigated using 51 winters of
re-analysis data and a set of coupled climate models. We
found large increases in the frequency of cold air outbreaks
(Figure 3) that coincide geographically with the regions of
mean temperature change (Figure 2). The probability of
CAOs was found to increase: (1) by 75% or more in some
regions of northern Asia throughout the life cycle of weak
vortex events (from the Precursor phase to the Decay phase),
(2) by 50% or more in some regions of Europe (from the
Onset phase to the Decline phase), and (3) by 50% or more
in the Peak phase off the east coast of North America.
Changes in the frequency of cold air outbreaks associated
with the stratosphere are therefore large compared to the
climatological incidence of CAOs. Such substantial changes
make this signal important for the long-range forecasting
of the likelihood of CAOs. If the signal is predictable, then
there will be an associated predictability of CAOs. However,
if it is unpredictable, then it represents an important limit
on the long-range predictability of CAOs.
A potential obstacle to the predictability of CAOs based
on the state of the stratospheric vortex is the fact that
many of the cold anomalies seen in Figure 3 occurred
2.5 1.5 0.5 0.5 1.5 2.5
0 0.25 0.5 0.75 1.25 1.5 1.75 2 2.25 2.5
Mature +
Growth +
Precursor +
Figure 5. (a) The 13-member climate model ensemble average 850 hPa
temperature and geopotential height anomalies, and (b) relative probability
of cold months, one month before, during and one month after WVMs.
(a) Plotting conventions as in Figure 2. (b) Plotting conventions as in
Figure 3.
before the SSW central dates and WVDs. The early CAOs in
Europe and Asia were associated with the perhaps clearest
precursor of stratospheric weak vortex events, a high-
pressure anomaly centred over the northwestern edge of
Eurasia in the Precursor, Onset and Growth phases. Although
its location changed with time, this positive height anomaly
persisted for all the phases and was confined to high latitudes
in the Atlantic sector. More work is therefore needed to
address the chain of cause and effect and to investigate
tropospheric precursors of weak vortex events, adding to
existing studies of troposphere stratosphere interactions
(Kuroda and Kodera, 1999; Chen et al., 2003; Polvani and
Waugh, 2004; Reichler et al., 2005; Scaife et al., 2005; Cohen
et al., 2007; Martius et al., 2009; Mukougawa et al., 2009;
Garfinkel et al., 2010).
We did not directly address the issue of cause and
effect of CAOs in this paper, but interestingly, we
found a hemisphere-wide pattern of lower-tropospheric
temperature signals both before and after weak vortex
events. In general, such temperature signals are associated
with pressure anomaly dipoles in the form of anomalous
ridges upstream (such as the precursory high anomaly over
northwest Eurasia) and anomalous troughs downstream of
the cold anomalies. Such patterns lead to changes to the
flow, and the resulting temperature advections may well
act as positive feedback mechanisms, as documented for the
negative phase of the surface NAM (Thompson and Wallace,
2000). The association between pressure anomaly dipoles
and CAOs is known from previous studies (Konrad, 1996;
Walsh et al., 2001; Chen et al., 2005; Takaya and Nakamura,
2005; Cellitti et al., 2006; Kolstad et al., 2009). It is quite
possible that some of the regional CAOs identified in this
paper are at least partly set up or sustained by cold air
advection, as part of the chain of events outlined by Konrad
Given the strong stratospheric link to many CAOs, it
of the stratosphere in climate models. However, parts of
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2010 Royal Meteorological Society and Crown Copyright. Q. J. R. Meteorol. Soc. 136: 886– 893 (2010)
892 E. W. Kolstad et al.
our analysis were repeated with an ensemble of 13 coupled
climate models. Somewhat surprisingly, considering that
many of these models have low model tops and poorly
resolved stratospheres (Cordero and Forster, 2006), the
model results corroborated the relationships between the
weak vortex events and the cold anomalies listed above. This
may indicate that the main aspects of the tropospheric
temperature developments during the life cycle of the
stratospheric weak vortex events are associated with internal
processes in the troposphere and lower stratosphere, as
suggested by Polvani and Waugh (2004).
We wish to thank the editors and two anonymous reviewers
the modelling groups, the Program for Climate Model
Diagnosis and Intercomparison (PCMDI) and the WCRP’s
Working Group on Coupled Modelling (WGCM) for
making available the WCRP CMIP3 multi-model dataset,
as well as NOAA/OAR/ESRL PSD for providing the
NCEP/NCAR re-analysis data. Erik Kolstad’s work was
funded by the Norwegian Research Council through its
International Polar Year programme and the project IPY-
THORPEX (grant number 175992/S30). Tarjei Breiteig’s
work was supported by the COMPAS project (grant number
165424), also funded by the Norwegian Research Council.
Adam Scaife was supported by the Joint DECC and Defra
Integrated Climate Programme – DECC/Defra (GA01101).
This is publication no. A259 from the Bjerknes Centre for
Climate Research.
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... The northern European regions of Scandinavia and the British Isles experience reduced precipitation, while central and southern Europe experience wetter-than-average conditions. In East Asia, SSWs can affect the East Asian winter monsoon (Deng et al., 2008): although China on average tends to experience milder conditions following an SSW (Lim et al., 2019), East Asia in general can see an increased risk of extreme cold air outbreaks Kolstad et al., 2010;Song et al., 2015). In general, SSWs are linked to extremes in surface climate (e.g. ...
... Recognising the limited observational sample, some studies have sought to increase their sample sizes by using free-running climate models (e.g. Garfinkel et al., 2010;Karpechko et al., 2017;Kolstad et al., 2010;Kolstad and Charlton-Perez, 2011;Maycock and Hitchcock, 2015), performing dedicated model experiments (e.g. de la Cámara et al., 2017;White et al., 2021), or using "ensembles of opportunity" from climate model experiments designed for other studies (e.g. White et al., 2019). ...
... High pressure or blocking, approximately over Eurasia (and often the polar cap), has often been found to be a precursor to SSWs in general (e.g. Cohen and Jones, 2011;Kolstad et al., 2010;Kolstad and Charlton-Perez, 2011;White et al., 2019). However, our results extend this by quantifying the impact of these regions as precursors of post-SSW negative NAO conditions. ...
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Sudden-stratospheric-warming (SSW) events are often followed by significant weather and climate impacts at the surface. By affecting the North Atlantic Oscillation (NAO), SSWs can lead to periods of extreme cold in parts of Europe and North America. Previous studies have used observations and free-running climate models to try to identify features of the atmosphere prior to an SSW that can determine the subsequent impact at the surface. However, the limited observational record makes it difficult to accurately quantify these relationships. Here, we instead use a large ensemble of seasonal hindcasts. We first test whether the hindcasts reproduce the observed characteristics of SSWs and their surface signature. We find that the simulations are statistically indistinguishable from the observations, in terms of the overall risk of an SSW per winter (56 %), the frequency of SSWs with negative NAO responses (65 %), the magnitude of the NAO responses, and the frequency of wavenumber-2-dominated SSWs (26 %). We also assess the relationships between prior conditions and the NAO response in the 30 d following an SSW. We find that there is little information in the precursor state to guide differences in the subsequent NAO behaviour between one SSW and another, reflecting the substantial natural variability between SSW events. The strongest relationships with the NAO response are from pre-SSW sea level pressure anomalies over the polar cap and from zonal-wind anomalies in the lower stratosphere, both exhibiting correlations of around 0.3. The pre-SSW NAO has little bearing on its post-SSW state. The strength of the pre-SSW zonal-wind anomalies at 10 hPa is also not significantly correlated with the NAO response. Finally, we find that the mean NAO response in the first 10 d following wave-2-dominated SSWs is much more strongly negative than in wave-1 cases. However, the subsequent response in days 11–30 is very similar regardless of the dominant wavenumber. In all cases, the composite mean responses are the result of very broad distributions from individual SSW events, necessitating a probabilistic analysis using large ensembles.
... This provides a piece of evidence in terms of IMMC branches for the existence of stratospheric downward impact on the surface weather, which has been highlighted by several studies that are aimed at exploring the potential usage of stratospheric variability in the extended-range or S2S forecasts (Baldwin & Dunkerton, 1999, 2001Baldwin et al., 2003Baldwin et al., , 2021Cai et al., 2016;Deng et al., 2008;J. Huang et al., 2021;Kidston et al., 2015;Kodera et al., 1990;Kolstad et al., 2010;Mitchell et al., 2013;Thompson et al., 2002;Thompson & Wallace, 2001;Wang & Chen, 2010;Waugh et al., 2017;White et al., 2021;Y. X. Zhang et al., 2022). ...
... In the midlatitudes, cold anomalies are mainly in the larger-in-size Eurasian sector (0°-120°E) and warm anomalies in the North American sector (30°W-120°W) as shown in Figures 7c and 7d. This is consistent with previous studies that found cold events often occur in Eurasia before a negative NAM event or a displacement-type SSW event (Kolstad et al., 2010;Lehtonen & Karpechko, 2016;Yu, Li, et al., 2022). Previous studies also indicated that the enhanced upward planetary waves mainly originate from East Asia, which can effectively weaken the stratospheric polar vortex (Andrews et al., 1987;Matsuno, 1971;Polvani & Waugh, 2004;Shen et al., 2023) and drive a strong ST60N event (Yu et al., 2023). ...
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Cold air outbreaks (CAOs) in northern winter can be attributed to the variations in the lower‐tropospheric equatorward cold branch (CB) of the isentropic meridional mass circulation (IMMC) across the subpolar latitudes. Understanding when and how the variations of CB are related to the stratosphere is crutial for improving the extended‐range forecast skills of CAOs in winter. Investigation on the vertical coupling of the stratospheric and tropospheric branches of IMMC suggests that the CB (or CAOs) exhibits two major timescales: sub‐monthly (10–30 days) and monthly‐to‐seasonal (>30 days). The CB can be modified by the stratospheric variability that is represented by the stratospheric poleward warm branch (WB‐ST) of the IMMC at monthly‐to‐seasonal timescales via the two‐way stratosphere‐troposphere coupling. At submonthly timescales, however, the upward one‐way coupling from the troposphere to the stratosphere dominates. While changes of the CB always precede those of the WB‐ST at both timescales, indicating the upward tropospheric wave forcing into the stratosphere, the downward impacts of the WB‐ST on the CB (particularly CAOs in North America) are observed only at monthly‐to‐seasonal timescales. Specifically, the accumulated effect of the monthly‐to‐seasonal mass transport by the poleward WB‐ST can be strong enough to dominate the Arctic total column airmass changes, producing not only a barotropic structure of the Northern Annular Mode but also direct changes to the low‐level ageostrophic zonal‐mean meridional flow. The anomalous low‐level ageostrophic flow, in turn, plays a greater role in intensifying (weakening) the equatorward CB following a stronger (weaker) WB‐ST than tropospheric wave activities.
... Such events are known as sudden stratospheric warmings (SSWs; Scherhag 1952, Butler et al 2015, Baldwin et al 2021. This pattern of anomalies can influence the surface over a period of a month or so (Baldwin and Dunkerton 2001), and may result in prolonged severe cold weather in certain regions (Domeisen andButler 2020, Hall et al 2022), particularly at higher latitudes over Eurasia (Kolstad et al 2010), as the eddy-driven jet and North Atlantic storm track shift southwards, with a more negative North Atlantic Oscillation (NAO) (Kidston et al 2015). The storm track shift can result in increased likelihood of storms entering the Mediterranean region, with consequent flooding (Afargan-Gerstman et al 2020). ...
... The storm track shift can result in increased likelihood of storms entering the Mediterranean region, with consequent flooding (Afargan-Gerstman et al 2020). In reanalysis data and model output, around 40%-50% of winter European cold spells were preceded by stratospheric dynamical disturbances (Kolstad et al 2010, Tomassini et al 2012. ...
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Sudden stratospheric warmings (SSWs) have been linked to surface temperature anomalies, but how these connect to changes in the likelihood of specific weather extremes and their associated weather patterns remains uncertain. While, on average, it is true that cold surface temperatures follow SSW events, particularly in Northern Europe, there is considerable event-to-event variability. Over the British Isles and Central Europe, only around 45% of SSWs are followed by a colder than average period and a negative phase of the North Atlantic Oscillation (NAO), cautioning against an over-generalised approach to surface anomalies associated with SSWs. Focussing on more hazardous weather, which in winter is associated with cold extremes, we use reanalysis data to consider how SSWs impact temperature-related hazards; namely the frequency of snowy days, frost days and spells of extreme cold weather in 12 major European cities. In general, SSWs are associated with an increased risk of snow across most of western Europe, and that this is particularly significant in milder, more maritime locations such as London where in reanalysis, snowfall days are 40% more likely after an SSW. However, there is considerable variation in surface temperature anomalies between SSW events; the third of SSWs with the warmest surface anomalies are statistically more likely to have a decreased risk of snow, frost and persistent cold spells compared with non-SSW time periods. These warmer events are associated with a different temperature anomaly pattern, which is consistent in both reanalysis data and large ensemble CMIP6 models. We further show that these warm surface temperature anomaly SSWs are becoming more frequent, a trend which is consistent with background global warming. The varied surface anomalies associated with SSWs highlights the need to study their impacts in a probabilistic sense, and motivates further work to enable better prediction of the impacts of a given event.
... As early as 1977, research indicated that circulation anomalies in the stratosphere could influence the troposphere [3]. Subsequent studies have further demonstrated that the stratosphere is not solely passively influenced by upward propagating anomalies from the troposphere as it can also exert an influence on tropospheric weather [1,[4][5][6]. The downward coupling of anomalies in the polar stratosphere plays a crucial role in driving low-frequency variations in tropospheric circulation and temperature, making it a source of predictability for surface weather and climate [7][8][9]. ...
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Fifty years of daily ERA5 reanalysis data are employed to investigate the linkages between the strength of the stratospheric polar vortex and the tropospheric polar vortex during the boreal winter. The strong coupling events, anomalies in both the stratospheric and tropospheric polar vortices, can be classified into four configurations, each representing the distinct characteristics of planetary wave vertical propagation and tropospheric circulation anomalies. The findings reveal the following patterns: (1) Strong stratospheric polar vortex and weak tropospheric polar vortex periods are associated with anomalous downward E-P flux from the stratosphere to the troposphere, predominantly induced by planetary waves 1 and 2. Warm anomalies occur along the North Atlantic coasts, while cold anomalies are evident over Eastern Europe and East Asia at the surface. (2) Weak stratospheric polar vortex and strong tropospheric polar vortex periods exhibit anomalous upward E-P flux in high latitudes, with dominant wave 1, and anomalous downward E-P flux in the middle latitudes, dominated by wave 2. Warm anomalies are observed over North America, Western Europe, and the northern side of the Gulf of Oman at the surface. (3) Strong stratospheric polar vortex and strong tropospheric polar vortex periods feature anomalous downward E-P flux in high latitudes, dominated by wave 1, and anomalous upward E-P flux in middle latitudes, with a wave 2 predominance. Warm anomalies prevail over Northeast Asia, Southern Europe, and North America at the surface. (4) Weak stratospheric polar vortex and weak tropospheric polar vortex periods display anomalous upward E-P flux in mid-to-high latitudes, predominantly with wave 1. In contrast to the tropospheric circulation anomalies observed in the third category, this pattern results in the presence of cold anomalies over Northeast Asia, Southern Europe, and North America.
... The increased incidence of major wintertime cold air outbreaks across the Northern Hemisphere, particularly over Eurasia (Kretschmer et al., 2018a), during either stratospheric weak vortex or SSW events has been a subject of much attention in recent years (Thompson et al., 2002;Kolstad et al., 2010;Kretschmer et al., 2018b), given the potential for significant advances in sub-seasonal predictive skill (Sigmond et al., 2013;Scaife et al., 2016). Intrinsic to such changes is the equatorward displacement of the tropospheric jet stream and associated storm tracks as the stratospheric jet weakens (Baldwin and 50 Dunkerton, 1999;Baldwin and Dunkerton, 2001;Domeisen et al. 2013;Kidston et al., 2015). ...
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Sudden stratospheric warmings (SSWs) are abrupt disturbances to the Northern Hemisphere wintertime stratospheric polar vortex that can lead to pronounced regional changes in surface temperature and precipitation. SSWs also strongly impact the distribution of chemical constituents within the stratosphere, but the implications of these changes for stratosphere-troposphere exchange (STE) and radiative effects in the upper troposphere-lower stratosphere (UTLS) have not been extensively studied. Here we show, based on a specified-dynamics simulations from the EMAC chemistry-climate model, that SSWs lead to a pronounced increase in high-latitude ozone just above the tropopause (>25 % relative to climatology), persisting for up to 50 days for the ~50 % events classified as downward propagating following Hitchcock et al. (2013). This anomalous feature in lowermost stratospheric ozone is verified from ozone-sonde soundings and using the Copernicus Atmospheric Monitoring Service (CAMS) atmospheric composition reanalysis product. A significant dipole anomaly (>±25 %) in water vapour also persists in this region for up to 75 days, with a drying signal above a region of moistening, also evident within the CAMS reanalysis. Resultant enhanced STE leads to a significant 5–10 % increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days. The signal also propagates to mid-latitudes leading to significant enhancements in UTLS ozone, and, of weakening strength, also in free tropospheric and near-surface ozone up to 90 days after the event. In quantifying the potential significance for surface air quality breaches above ozone regulatory standards, a risk enhancement of up to a factor of 2 to 3 is calculated following such events. The chemical composition perturbations in the Arctic UTLS result in radiatively-driven Arctic stratospheric temperature changes of around 2 K. An idealised sensitivity evaluation highlights the changing radiative importance of both ozone and water vapour perturbations with seasonality. Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality. Accurate representation of UTLS composition (namely ozone and water vapour), through its effects on local temperatures, may also help improve numerical weather prediction forecasts on sub-seasonal to seasonal timescales.
... As a result, the anomalous signals in the stratospheric polar vortex can exert important downward impacts on weather and climate in the extratropical troposphere. For instance, during the 1-2 months after a weaker stratospheric polar vortex event such as sudden stratospheric warming (SSW) and negative stratospheric northern annular mode event, below-normal temperatures tend to occur more frequently over the Northern Hemispheric continents [24,[27][28][29][30][31][32][33][34][35][36][37][38]. In addition, the polar temperature related to the SPV in the late winter months is closely related to the formation of polar stratospheric clouds and thus affects the Arctic ozone depletion [39][40][41][42]. ...
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The year-to-year varying annual evolutions of the stratospheric polar vortex (SPV) have an important downward impact on the weather and climate from winter to summer and thus potential implications for seasonal forecasts. This study constructs a parametric elliptic orbit model for capturing the annual evolutions of mass-weighted zonal momentum at 60° N (MU) and total air mass above the isentropic surface of 400 K (M) over the latitude band of 60–90° N from 1 July 1979 to 30 June 2021. The elliptic orbit model naturally connects two time series of a nonlinear oscillator. As a result, the observed coupling relationship between MU and M associated with SPV as well as its interannual variations can be well reconstructed by a limited number of parameters of the elliptic orbit model. The findings of this study may pave a new way for short-time climate forecasts of the annual evolutions of SPV, including its temporal evolutions over winter seasons as well as the spring and fall seasons, and timings of the sudden stratospheric warming events by constructing its elliptic orbit in advance.
... Here, we update these composite results with 46 SSW events based on ERA5 reanalysis data, as shown in Fig. 1. The canonical tropospheric response to SSW is a negative phase of the NAM, corresponding to an equatorward shift of storm tracks, severe cold-air outbreaks, and heavy snowfall over most of the midlatitude Northern Hemisphere (Baldwin and Dunkerton, 2001;Charlton and Polvani, 2007;Kolstad et al., 2010;Wang and Chen, 2010;Yu et al., 2015Yu et al., , 2018Kretschmer et al., 2018;Huang and Tian, 2019;Domeisen and Butler, 2020;Huang et al., 2021). ...
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In this review, instead of summarizing all the advances and progress achieved in stratospheric research, the main advances and new developments in stratosphere-troposphere coupling and stratospheric chemistry-climate interactions are summarized, and some outstanding issues and grand challenges are discussed. A consensus has been reached that the stratospheric state is an important source of improving the predictability of the troposphere on sub-seasonal to seasonal (S2S) time scales and beyond. However, applying stratospheric signals in operational S2S forecast models remains a challenge because of model deficiencies and the complexities of the underlying mechanisms of stratosphere-troposphere coupling. Stratospheric chemistry, which controls the magnitude and distribution of many important climate-forcing agents, plays a critical role in global climate change. Convincing evidence has been found that stratospheric ozone depletion and recovery have caused significant tropospheric climate changes, and more recent studies have revealed that stratospheric ozone variations can even exert an impact on SSTs and sea ice. The climatic impacts of stratospheric aerosols and water vapor are also important. Although their quantitative contributions to radiative forcing have been reasonably well quantified, there still exist large uncertainties in their long-term impacts on climate. The advances and new levels of understanding presented in this review suggest that whole-atmosphere interactions need to be considered in future for a better and more thorough understanding of stratosphere-troposphere coupling and its role in climate change.
... The breakdown of the stratospheric vortex exerts a "downward influence" on tropospheric circulation (Baldwin & Dunkerton, 2001;Baldwin et al., 2003;Hitchcock & Simpson, 2014;Kidston et al., 2015). The midlatitude jet and storm track shift equatorward, bringing extreme cold spells and other anomalous weather to nearby regions (Kolstad et al., 2010;Kretschmer et al., 2018). For example, King et al. (2019) documents the impact of an SSW on extreme winter weather over the British Isles, the so-called "Beast from the East" in February 2018. ...
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Extreme weather events have significant consequences, dominating the impact of climate on society. While high‐resolution weather models can forecast many types of extreme events on synoptic timescales, long‐term climatological risk assessment is an altogether different problem. A once‐in‐a‐century event takes, on average, 100 years of simulation time to appear just once, far beyond the typical integration length of a weather forecast model. Therefore, this task is left to cheaper, but less accurate, low‐resolution or statistical models. But there is untapped potential in weather model output: despite being short in duration, weather forecast ensembles are produced multiple times a week. Integrations are launched with independent perturbations, causing them to spread apart over time and broadly sample phase space. Collectively, these integrations add up to thousands of years of data. We establish methods to extract climatological information from these short weather simulations. Using ensemble hindcasts by the European Center for Medium‐range Weather Forecasting archived in the subseasonal‐to‐seasonal (S2S) database, we characterize sudden stratospheric warming (SSW) events with multi‐centennial return times. Consistent results are found between alternative methods, including basic counting strategies and Markov state modeling. By carefully combining trajectories together, we obtain estimates of SSW frequencies and their seasonal distributions that are consistent with reanalysis‐derived estimates for moderately rare events, but with much tighter uncertainty bounds, and which can be extended to events of unprecedented severity that have not yet been observed historically. These methods hold potential for assessing extreme events throughout the climate system, beyond this example of stratospheric extremes.
... These time scales are considered to be a so-called predictability "grey zone" for which the predictions are highly inaccurate [6,7]. The extreme states of the stratospheric polar vortex (SPV) affect the location of the Northern Hemisphere storm tracks on the periods from 10 days to 2 months [2,[8][9][10][11][12][13][14][15][16][17][18][19], thus providing potential predictability for mid-latitudinal states of the troposphere. This causes diagnostics and understanding the mechanisms of SPV variability to be crucial for predications of the troposphere dynamics. ...
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The deterministic forecast of the stratospheric polar vortex intensity (iSPV) is limited by 2 weeks, but it can be probabilistically predicted for a longer period due to low-frequency components of the climate system, such as large-scale sea surface temperature anomalies (SSTAs) (e.g., El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO)). For a qualitative and quantitative assessment of the effect of the large-scale Pacific SSTAs on the iSPV anomalies formation, idealized model experiments were carried out using the Isca platform. There is no statistically significant response of the SPV dynamics to the SSTAs corresponding to PDO phases, but they noticeably correct the effect of ENSO modes when added to it. The effect of the El Niño (EN) and La Niña (LN) events with neutral PDO phases on the iSPV is asymmetric; in the “single” EN experiment the vortex is 40% weaker relative to the control values, and, in the “single” LN, the SPV is weakened by no more than 20%. When EN accompanied with the positive PDO phase, iSPV is reduced by 58%. When the negative PDO phase is added, the EN effect is significantly weakened. The LN effect is weakened by both positive and negative PDO phases.
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The Bergen Climate Model (BCM) is a fully-coupled atmosphere-ocean-sea-ice model that provides state-of-the-art computer simulations of the Earth's past, present, and future climate. Here, a pre-industrial multi-century simulation with an updated version of BCM is described and compared to observational data. The model is run without any form of flux adjustments and is stable for several centuries. The simulated climate reproduces the general large scale circulation in the atmosphere reasonably well, except for a positive bias in the high latitude sea level pressures distribution. Also, by introducing an updated turbulence scheme in the atmosphere model a persistent cold bias has been eliminated. For the ocean part, the model drifts in sea surface temperatures and salinities are considerably reduced compared to earlier versions of BCM. Improved conservation properties in the ocean have contributed to this. Furthermore, by choosing a reference pressure at 2000 m and including thermobaric effects in the ocean model, a more realistic meridional overturning circulation is simulated in the Atlantic Ocean. The simulated sea-ice extent in the Northern Hemisphere is in general agreement with observational data except for summer where the extent is somewhat underestimated. In the Southern Hemisphere, large negative biases are found in the simulated sea-ice extent. This is partly related to problems with the mixed layer parametrization, causing the mixed layer in the Southern Ocean to be too deep, which in turn makes it hard to maintain a realistic sea-ice cover here. However, despite some problematic issues, the pre-industrial control simulation presented here should still be appropriate for climate change studies requiring multi-century simulations.
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The authors estimate the change in extreme winter weather events over Europe that is due to a long-term change in the North Atlantic Oscillation (NAO) such as that observed between the 1960s and 1990s. Using ensembles of simulations from a general circulation model, large changes in the frequency of 10th percentile temperature and 90th percentile precipitation events over Europe are found from changes in the NAO. In some cases, these changes are comparable to the expected change in the frequency of events due to anthropogenic forcing over the twenty-first century. Although the results presented here do not affect anthropogenic interpretation of global and annual mean changes in observed extremes, they do show that great care is needed to assess changes due to modes of climate variability when interpreting extreme events on regional and seasonal scales. How changes in natural modes of variability, such as the NAO, could radically alter current climate model predictions of changes in extreme weather events on multidecadal time scales is also discussed.
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The variability of both the stationary planetary wave activity and the East Asian winter monsoon is strongly associated with the thermal contrast between oceans and landmasses. In this study, we explore the interannual relationship between the monsoon and the wave activity defined by an index of the difference in the divergence of Eliassen-Palm flux between 50°N at 500 hPa and 40°N at 300 hPa. It is found that, compared to the winters of low wave activity, the equatorward propagation of planetary waves in the middle and upper troposphere is stronger in the high wave activity winters. During these high activity winters, the upward wave propagation from the troposphere into the stratosphere becomes weaker. This is accompanied by a smaller perturbation in the polar vortex, which tends to be colder and stronger. In the meantime, the East Asian westerly jet stream, the East Asian trough, the Siberian high, and the Aleutian low all become weaker apparently. In particular, the weakening of the Siberian high and the Aleutian low decreases the northeasterly wind over East Asia, leading to a warming condition in the region especially in northeastern Asia. A further analysis reveals that the zonal wavenumber-2 pattern of planetary waves contributes dominantly to the variability of the East Asian winter monsoon.
The leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep, zonally symmetric or "annular" structures, with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 45 latitude. The structure and dynamics of the Southern Hemisphere (SH) annular mode have been extensively documented, whereas the existence of a Northern Hemisphere (NH) mode, herein referred to as the Arctic Oscillation (AO), has only recently been recognized. Like the SH mode, the AO can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields. In this paper the structure and seasonality of the NH and SH modes are compared based on data from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis and supplementary datasets. The structures of the NH and SH annular modes are shown to be remarkably similar, not only in the zonally averaged geopotential height and zonal wind fields, but in the mean meridional circulations as well. Both exist year-round in the troposphere, but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave-mean flow interaction: midwinter in the NH and late spring in the SH. During these "active seasons," the annular modes modulate the strength of the Lagrangian mean circulation in the lower stratosphere, total column ozone and tropopause height over mid- and high latitudes, and the strength of the trade winds of their respective hemispheres. The NH mode also contains an embedded planetary wave signature with expressions in surface air temperature, precipitation, total column ozone, and tropopause height. It is argued that the horizontal temperature advcction by the perturbed zonal-mean zonal wind field in the lower troposphere is instrumental in forcing this pattern. A companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades.
Marine ecosystems are undergoing rapid change at local and global scales. To understand these changes, including the relative roles of natural variability and anthropogenic effects, and to predict the future state of marine ecosystems requires quantitative understanding of the physics, biogeochemistry and ecology of oceanic systems at mechanistic levels. Central to this understanding is the role played by dominant patterns or modes of atmospheric and oceanic variability, which orchestrate coherent variations in climate over large regions with profound impacts on ecosystems. A leading pattern of weather and climate variability over the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO refers to a redistribution of atmospheric mass between the Arctic and the subtropical Atlantic, and swings from one phase to another produce large changes in surface air temperature, winds, storminess and precipitation over the Atlantic as well as the adjacent continents. The NAO also affects the ocean through changes in heat content, gyre circulations, mixed layer depth, salinity, high latitude deep water formation and sea ice cover. Thus, indices of the NAO have become widely used to document and understand how this mode of variability alters the structure and functioning of marine ecosystems. There is no unique way, however, to define the NAO, and so several approaches will be discussed including nonlinear techniques that reveal spatial asymmetries between different phases of the NAO that are likely important for ecological studies. It also follows that there is no universally accepted index to describe the temporal evolution of the NAO. Several of the most common measures are presented and compared. All reveal that there is no preferred time scale of variability for the NAO: large changes occur from one winter to the next and from one decade to the next. There is also a large amount of within-season variability in the patterns of atmospheric circulation of the North Atlantic, so that most winters cannot be characterized solely by a canonical NAO structure. A better understanding of how the NAO responds to external forcing, including sea surface temperature changes in the tropics, stratospheric influences, and increasing greenhouse gas concentrations, is crucial to the current debate on climate variability and change.
Reanalysis output for 1948-99 is used to evaluate the temporal distributions, the geographical origins, and the atmospheric teleconnections associated with major cold outbreaks affecting heavily populated areas of middle latitudes. The study focuses on three subregions of the United States and two subregions of Europe. The cold outbreaks affecting the United States are more extreme than those affecting Europe, in terms of both the regionally averaged and the local minimum air temperatures. There is no apparent trend toward fewer extreme cold events on either continent over the 1948-99 period, although a long station history suggests that such events may have been more frequent in the United States during the late 1800s and early 1900s. The trajectories of the coldest air masses are southward or southeastward over North America, but westward over Europe. Subsidence of several hundred millibars is typical of the trajectories of the coldest air to reach the surface in the affected regions. Sea level pressure anomalies evolve consistently with the trajectories over the 1-2 weeks prior to the extreme outbreaks, and precursors of the cold events are apparent in coherent antecedent anomaly patterns. Negative values of the North Atlantic oscillation index and positive anomalies of Arctic sea level pressure are features common to North American as well as European outbreaks. However, the strongest associated antecedent anom- alies of sea level pressure are generally shifted geographically relative to the nodal locations of the North Atlantic and Arctic oscillations.